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This invention relates to improvements in runway localizer antennas for the Instrument Landing System (ILS). Many localizer antenna systems employ an array of radiating elements, fed by coaxial cables from a centrally located distribution unit, (DU). The environment of modern airports has required that localizer antenna systems have larger apertures, with more elements, in order to produce the narrow beams needed to reduce multi-path interference. This, in turn, causes increases in the cost and complexity of the DU and associated coaxial feed cables. This invention uses features of an earlier U.S. Pat. No. 3,577,197, May 4, 1971, Watts, Jr., xe2x80x9cSlotted Cable Localizer Antenna,xe2x80x9d (Ref.1) to replace the central DU with a low-loss rigid copper transmission line running the entire length of the array. The radiating elements are fed from short cables through adjustable capacitors connected periodically along the rigid line. The result is a structure of reduced cost and complexity.
This antenna system radiates simultaneous sum and difference patterns carrying standard ILS reference (CSB) and deflection (SBO) components of the transmitted signal. The signal components from an ILS transmitter are supplied through an rf bridge (hybrid) to both ends of a distributor structure. Optional isolators inserted in the coaxial feed lines improve the impedance presented to the transmitter. An object of the invention is to provide a localizer antenna system having improved efficiency and ease of construction. This is accomplished through the particular arrangement of parts, including a rigid copper coaxial transmission line running the entire length of the array, with capacitive taps feeding the individual elements.